Heave compensator

ABSTRACT

The dead end of a drilling cable is attached to a cable drum operably connected to a cylinder/piston assembly, the cable drum being spirally shaped to keep the tension in the drilling cable independent of the position of the piston.

United States-Patent [191 van Daalen HEAVE COMPENSATOR [75] inventor Francois van Daalen, The Hague,

Netherlands [73] Assignee: Shell Oil Company, New York, NY. [22] Filed: Apr. 20, 1971 [21] Appl. No.: 135,667

[30] Foreign Application Priority Data Apr. 30, 1970 Great Britain 20,930/70 Dec. 16, 1970 Great Britain 59,719/70 [52] ILLS. C1 254/172, 175/5 [51] Int. Cl B6611 H48 [58] Field of Search 254/172, 173; 187/20, 23; 166/.5; 175/5-7 [56] References Cited UNITED STATES PATENTS 2,370,029 2/1945 Gillespie 187/23 UX [u] 3,743,249 [451 July 3,1973

11/1964 Kammerer 175/5 X 9/1965 Parks 254/172 vFOREIGN PATENTS OR APPLICATIONS 1,329,220 4/1963 France 254/172 Primary Examiner-Evon C. Blunk Assistant Examiner-Merle F. Maffei Attorney-Theodore E. Bieber et al.

[5 7} ABSTRACT The dead end of a drilling cable is attached to a cable drum operably connected to a cylinder/piston assembly, the cable drum being spirally shaped to keep the tension in the drilling cable independent of the position of the piston.

9 Claims, 3 Drawing Figures Patented July 2 Sheets-Sheet 2 FIG. 2 42 63 L 49. fan 55 o I o V v FIG. 3

I T "E T 52 58 55 53 50 F. Van Dcrczlen W INVENTOR L +AL HEAVE COMPENSATOR BACKGROUND OF THE INVENTION tance between the ends of the cable, chain or flexible conduit or a variation in the distance between pulleys or sheaves over which the cable, chain or flexible conduit is guided.

. Such apparatus may be of use in drilling boreholes in the sea bottom from a ship or other structure floating on the water. Due to the action of the waves and the vertical displacements of the drill ship resulting therefrom, the tension in the drilling cable and in the guidelines leading from the ship to the submerged wellhead varies and it is often necessary to control the magnitude of the load exerted on each cable or to make arrangements to prevent overloading of the cables.

To this end, use can be made of constant-tension winches which are provided with control equipment for measuring the load on the cable and controlling the operation of the winch as a result thereof. A disadvantage of such a system is that it needs a prime mover which has to stand by continuously. Moreover, the winches are provided with'brakes which have to be engaged and disengaged each time a correction in the load is required, and as a result, they wear and require constant supervision.

Another way of keeping the load or tension in a cable constant is to connect one end of the cable to the piston rodof a piston/cylinder arrangement and maintain a constant pressure within the cylinder on one side of the piston by using a pressure detector and feeding the difference between the desired value and the measured value of thepressure in the cylinder to a controller for actuating a valve arranged in an air conduit leading from a pressure source to the cylinder and a valve arranged in a bleeding conduit connected to this cylinder. A disadvantage of this system is that rapidly occurring variations inthe load of the cable cannot be quickly compensated; this being particularly true when it is required to compensate for an increase in load.

BRIEF SUMMARY OF THE INVENTION Accordingly, this invention is directed to an apparatus suitablefor maintaining a constant tension in, or preventing overloading of a cable (or chain or flexible conduit), the apparatus being inexpensive, simple in construction, reliable in operation and requiring minimum maintenance.

According to the invention, an apparatus for maintainingv a constant tension in, or preventing overloading of a cable comprises a cylinder/piston arrangement having a piston connected to a cable drum through a transmission suitable to convert longitudinal displacements of the piston rod into rotation of the drum around its central axis, the drum having supporting surface for the cable, which supporting surface has a nonconstant radius, one end of the cable being connected to the drum and the part of the cable adjacent to said end passing over the supporting surface.

The supporting surface for the cable may be in the shape of a conical screw or a spiral. A source of high pressure fluid is connected to the cylinder/piston arrangement. The combination of a spiral supporting surface and a cylinder/piston arrangement supplied with high pressure fluid produces a constant tension in the cable.

' BRIEF DESCRIPTION OF THE DRAWING The invention will now be described by way of example with reference to the drawing in which:

FIG. 1 shows a vertical section through part of a drill ship and drilling rig incorporating this invention;

FIG. 2 shows schematically a travelling block of a drilling rig combined with equipment according to the invention; and

FIG. 3 shows the combination of the equipment shown in FIG. 2, but with the equipment in another position.

DESCRIPTION OF A PREFERRED EMBODIMENT The drill ship 1 which is only partly shown comprises an opening 2 through which the drill string 3 extends, which drill string is used for drilling the borehole 4 in the formation 5 below the body of water 6 having a level 7.

The drill string 3 passes through. wellhead equipment when entering the borehole 4. For the sake of simplicity only the blow-out preventer 8 of this wellhead equipment has been shown.

This blow-out preventer 8 is used for closing the passage through the annular space 9 around the drill string 3 by means of rams l0 operated by pistons 11 arranged in cylinders 12. The pistons are actuated by pressure fluid guided to the cylinder spaces by pressure conduits l3 and 13 leading from the ship 1 to the cylinders 12. Since various types of blow-out preventers are known by those skilled in the art, no further detailsof the blow-out preventer 8 are given. The drill string 3 is suspended froin the derrick 14 on the ship 1 by means of the drilling cable 15 which is guided over the sheaves of the crown block 16 supported from the top of the derrick l4 and the sheaves of the travelling block 17 carrying the drilling hook l8. Theswivel 19 connected to the top of the drill string 3 is suspended from the hook18.

One end of the drilling cable 15 is attached to the hoisting drum 20 of drawworks (not shown). The other end of the cable 15 passes over the supporting surface of the cable drum 21, mounted on a shaft (not shown) which is rotatably arranged with respect to the central axis thereof. The shaft carries a pinion 22 co-operating with a rack 23 connected to the piston rod 24. Rod 24 is connected to the piston 25 suitably arranged in the cylinder 26 which is mounted stationary on the deck of the ship 1.

The supporting surface of cable drum 21 has a nonconstant radius and has the shape of a conical screw. A source 28 of pressure gas is further provided, this source communicating through a conduit 29 and valve 30 with one of the cylinder spaces. A pressureindicator 31 suitable to measure and indicate the pressure in this cylinder space is mounted on the conduit 29 at a location between the valve 30 and the cylinder 26.

Operation The operation of the apparatus is as follows.

By opening the valve 30, the gas pressure inside the left-hand side of the cylinder 26 is raised to value sufficiently high to keep the piston 25 in the position as shown in FIG. 1 against the force exerted on the piston rod 24 by the end of the drilling cable 15 by the cable drum 21, the pinion 22 and the rack 23. The valve 30 is closed when the desired pressure prevails in the lefthand side of the cylinder 26.

In this situation, the end of drilling cable 15 attached to the cable drum 21 is kept stationary, even when the hoisting drum 20 of the drawworks having the other end'of this cable 15 connected thereto, is operated to displace the drill string 3 in vertical direction. Vertical displacement of the ship as a result of wave action will not displace the piston 25 relative to the cylinder 26, provided that the pressure as exerted in the left-hand compartment of the cylinder 26 has been chosen sufficiently high. When it is desired to close the blow-out preventer 8 for safety reasons, a high pressure is exerted on the fluid in the conduits 13 for displacing the rams 10, which rams on closing clamp the drill string 3 therebetween. Upward movement of the ship after closing of the blow-out preventer 8 will increase the load on the drilling cable 15 until the load in the end thereof attached to the cable drum 21 is sufficiently high to move the piston 25 to the left against the gas pressure prevailing in the left side cylinder space. By

, this displacement of the piston, the pressure inside the cylinder space will increase owing to the decrease in volume of this space. At the same time, however, the longitudinal movement of the piston rod 24 will, as this rod is connected to the cable drum 21 via a rack and pinion system 22, 23, rotate the cable drum 21, thereby partly unwinding the cable 15 from the drum 21. At the same time the radius at which the cable 15 contacts the drum 2 1, is increased. With a suitable choice of the shape of the supporting surface of the cable drum 21 the increase in radius willcompensate the increase in pressure in the cylinder space in such a way that the load exerted on the end of the drilling cable 15 remains substantially constant.

c total length of stroke of the piston in the cylinder, s displacement of piston from its end position, p, pressure in cylinder space in end position of piston, p, pressure in cylinder when piston has been displaced over a distance F force exerted by gas pressure on piston in end position thereof, F, force exerted by gas pressure on piston when piston has been displaced over distance L load exerted on cable end, a radius of pinion, r radius at which cable contacts cable drum when piston is in its end position, r, radius at which cable contacts cable drum when piston has been displaced over a distance d: angle of rotation of pinion when piston is displaced over a distance and assuming that the gas in the cylinder can be described using the Ideal Gas Law (isothermal case), then L Fo/(l-s/c) a/ By choosing R r /(l s/c) or r ll-(alp/c) (5) the load on the cable remains constant independent of the position of the piston 25 within the cylinder 26..

It will be appreciated that the shape of the supporting surface of the cable drum 21 is not limited to the one indicated by formula (5). Deviations may be introduced e.g., for compensating friction variations occurring during the stroke of the piston, or for compensating the variation in the angle under which the cable is fed to the cable drum.

A suitable way of designing the shape of the surface of cable drum 21 is by replacing the cable drum 21 by a drum having a constant radius and measuring the load on'cable in the various positions of the piston 25 within the cylinder 26. By the load/displacement diagram thus obtained, the exact shape of the drum 21 can be calculated, wherein the particular circumstances under which the gas is compressed, and the dynamic behavior of the system have been taken into account.

In the example as shown, there is incorporated an apparatus according to the invention for maintaining a constant load on the guideline 32 extending between the ship 1 and the wellhead comprising the blow-out preventer. Since such a guideline is known to those skilled in the art, it is not described here in detail.

The upper part of the guideline 32 is passed over a sheave 33 and the end thereof is connected to a cable drum 34 having a supporting surface with a nonconstant radius. This surface may be in the form of a conical screw or if only part of the surface as shown is used, have the shape of a spiral. The arrangement comprising a rack and pinion and a cylinder/piston system is similar to the one described hereinabove.

It will be appreciated that the application of the present invention is not limited to apparatuses using rack and pinion as a means for converting longitudinal displacements of the piston into rotation of the cable drum. Any other type of transmission suitable for the purpose may be applied as well. Such transmission may be formed by a roller chain connected at one end thereof to the piston 25 (or the piston rod 24) and guided over a sprocket wheel mounted on the shaft of the cable drum. Suitable guide wheels are provided to press the chain to the sprocket wheel. If desired, at least two roller chains may be used parallel to each other.

Further, the application of the apparatus according to the present invention is not limited to keeping the load on the cable constant or preventing overloading of such a cable. The apparatus may also'be used for maintaining a constant load on or preventing overloading of a chain or flexible conduit.

If the length of cable which is to be wound and unwound from the cable drum is relatively short, the supporting surface of the cable drum need not be in the shape of a conical screw, but may be shaped as a spiral. As the cable part passing over the spirally shaped supporting surface is in a flat plane, the cable may be replaced by a flexible member which can be bent in one direction only, e.g., a roller chain.

The cylinder 26 may be filled with liquid instead of gas, and communicate with a high pressure vessel which is partially filled with liquid.

Instead of the piston, the cylinder of the piston cylinder arrangement may be connected to the cable drum, whereas the piston is connected via the piston rod to a fixed point of the ship.

FIGS. 2 and 3 of the drawing show an application of the apparatus according to the invention in combination with a travelling block of a drilling rig.

The travelling block 40 is suspended from the drilling cable 41, which in its turn is suspended from a fixed block (notshown) of a drilling rig (also now shown). Pneumatic cylinders 42 and 43 are connected to each other by means of connecting plates (only one of which indicated by reference numeral 44 being shown in the drawing), and to the travelling block 40 by a pin 45 passing through openings between the plates 44 and an eye piece 46 of the travelling block 40.

The cylinders 42 and 43 have pistons 47 and 48 respectively slidably arranged therein, these pistons being coupled to piston rods 49 and 50 respectively.

, The'cylinders 42 and 43 are in fluid communication with reservoirs 51 and sl'respectively, which can be filled with high-pressure gas via valve-controlled conduits 52 and 53 respectively.

The lower ends of the piston rods 49 and 50 are mutually connected by a bridge piece 54, to which one end of the cable 55 is connected at point 56 thereof. The other end of this cable 55 is wound on a circular drum or sheave 57 and connected thereto.

A curved drum or sheave 58 is coupled to the circular sheave 57, the combination of these two sheaves being rotatably mounted on a shaft 59, which is carried at the ends thereof by the connecting plates 44. The sheave 58 has a spiral-shaped surface and has one end of a cable 60 connected thereto at the point 61. The other end of the cable 60 is connected to a point 62 of one of the connecting plates 44.

A sheave 63 provided with means 64 suitable to carry a load L is carried by the 'cable 60.

The operation of the embodiment of the invention shown in FIGS. 2 and 3 will not be described.

When the sheave 63 is loaded by a load L, the position of the pistons 47, 48 will be, as shown in FIG. 2. In this case the gas pressure prevailing in the cylinders 42, 43 is sufficient to maintain the pistons 47, 48 in their lowest position relative to the cylinders 42, 43, against the force exerted in the cable 55 under influence of the load L exerted on the sheave 63.

However, an increase of this load L by A L will increase the force in the cable 55 sufficiently to become slightly larger thanthe sum of the forces exerted by the gas on the pistons 47 and 48. The pistons will then be lifted and the sheave 63 will descend. Since the spiralshaped sheave 58 over which one end of the cable 60 passes will then be rotated in the direction indicated by the arrow 65 in FIG. 2, the momentum exerted on the sheave 58 by the cable 60 will increase with the increase of the radius of the sheave 58 at the point thereof where the cable 60 is tangential to this sheave. As, however, the sheave 57 which is. rotated simultaneously with the sheave 58 since being coupled thereto, has a constant radius, the force exerted in the cable will increase as a result of this rotation of the sheave 58. The spiral shape of the supporting surface of the sheave 58 is chosen such that the increase of the force exerted in the cable 55 when being wound on the sheave 57 is sufficient to compensate the increase of the force exerted by the gas on the pistons 47, 48 when these pistons are lifted in the cylinders 42, 43 by the cable 55 which is being wound onto the sheave 57.

Thus, the force exerted in the cable 60 by the load L A L will always be in equilibrium with the sum of the forces exerted by the gas on the pistons 47, 48, provided that these pistons 47, 48 are not in one of their end positions in the cylinders 42, 43.

The equipment according to FIGS. 2 and 3 can be used for maintaining a constant weight on bit when drilling a hole into the earths surface. The drill string is then suspended from the sheave 63 by means 64. During drilling the travelling block is lowered at a rate sufficient to maintain the pistons 47, 48 somewhere between theirend-positions relative to the cylinders 42, 43. The load suspended from the sheave 63 will then be constant and equal to L A L, as long as the pistons 47, 48 are not in one of their end-positions.

When drilling is carried out from a ship by means of the equipment according to FIG. 2, displacements of the ship in a vertical sense as a result of wave action will not influence the weight on bit. Further, if a blow-out preventer situated on the wellhead of the hole which is being drilled, is closed, and the drill string is clamped between the rams of this blow-out preventer, no overload of the drill string can take place as a result of the wave action displacing the ship in a vertical sense, since the load on the drill string cannot increase above the value-+ A L as long as the pistons 47, 48 are not in one of their end positions.

It will be understood that the equipment according to the invention and shown in FIGS. 2 and 3'can also be applied for maintaining a constant weight on the bit when drilling a hole on land. By lowering the travelling block at a rate sufi'icient to maintain the pistons somewhere between their end positions in the cylinders the load on the sheave 63 will remain constant.

Variations in resistance met bythe drill, which result in variations in the drilling rate when the bit passes through the various layers of the underground formations will not influence the weight exerted on the bit, as long as the pistons 47,48 are not in one of their end positions.

It will be appreciated that several amendments may be introduced in the schematic design of the embodiment of the invention as shown in FIGS. 2 and 3. The spiral-shaped sheave 58 may be replaced by a cable drum having a supporting surface for the cable 60, which surface has the form of a conical screw. Also, sheave 57 may be replaced by a cable drum around which more than one loop of cable 55 can be wound. Two (or even more than two) systems, each consisting of the sheaves 57, 58, the cables 55, 60 and the sheave 63, may be used. The sheaves 63 are then combined to form a travelling block with common means 64 to suspend a load therefrom. By applying two (or any other even number) of these systems, the unbalance of the system as shown in FIG. 2 resulting from the asymmetric location of the points 56 and 62 with respect to the central axis of the block 40, can easily be compensated by locating the corresponding points of the other system at the other side of the. central axis of the block 40.

Alternatively, two systems, each comprising a set of sheaves 57, S8 and a cable 55 may be used. A single sheave 63 is then applied, carried by a cable which is at one end thereof connected to the spiral-shaped sheave 58 of the one system, and at its other end to the spiral-shaped sheave of the other system. The two spiral-shaped sheaves may be rotatably mounted on a common shaft to move in opposite directions when the pistons 47, 48 are displaced relative to the cylinders 42,

If desired,'the sheave 57 may be replaced by a sheave having a non-constant radius, whereas the sheave 58 may be replaced by a sheave with a constant radius. By a suitable choice of the non-constant radius, the same effect can be reached as with the embodiment as shown in FIGS. 2 and 3.

It will further even be possible to provide the sheave 57 as well as the sheave S8 with a non-constant radius. The cable-supporting surfaces of these sheaves are then designed such that when the radius of the point where cable 60 is tangential to the sheave 58 increases on rotation of the sheaves 57, 58 in one direction, the radius of the point where the cable 55 is tangential to the sheave 57 will decrease at the same time.

The cables 60 and 65 may be replaced b chains. If desired, the sheaves 57 and 58 may be replaced by sprocket wheels. If necessary, suitable guide wheels may be used for pressing the chains to the sprocket wheels.

It will be understood that the apparatus according to the invention may also be used for suspending the fixed block of a drilling cable therefrom. The apparatus shown in FIG. 2, with the exception of the travelling block 40, may then be placed on the water table of a derrick by connecting the cylinders 42 43 to this water table. The fixed block of the drilling cable system is then suspended from the sheave 63, and the travelling block is provided with a hook for supporting the drill string or other equipment used in the drilling operation.

I claim as my invention:

1. An apparatus for maintaining a constant tension in a cable, said apparatus comprising:

a cylinder/piston assembly supplied with energy from an external source;

a cable drum, said drum being mounted for rotation about a central axis and having a supporting surface for the cable, said surface having a nonconstant radius, one end of the cable being connected to the drum and the part of the cable adjacent to said end passing over the supporting surface of the drum; and

a transmission means connecting said piston to the cable drum, said transmission means being suitable to convert longitudinal displacements of the piston into rotation of the cable drumabout its central axis.

2. The apparatus of claim 1 wherein the supporting surface is in the shape of a spiral.

3. The apparatus of claim 1 wherein the energy source is high pressure gas.

4. The apparatus of claim 1 wherein said apparatus is mounted on a floating structure that moves vertically in response to wave action and said cable extends from said floating structure to a position that does not move in response to wave action.

5. An apparatus for maintaining a constant tension in a cable, said apparatus comprising:

a first drum having suitable means for connecting one end of the cable thereto, said first drum having a cable supporting surface having a non-constant radius;

a cylinder/piston assembly supplied with energy from an external source;

a transmission means connecting the first drum and piston of said cylinder/piston assembly, said transmission means comprising a second drum coupled to the first drum to rotate in combination there- I with; and

a cable coupled between the piston and the second drum to rotate said second drum when said piston moves.

6. The apparatus of claim 5 wherein the supporting surface is in the shape of a spiral.

7. The apparatus of claim 5 wherein the energy source is high pressure gas.

8. An apparatus for maintaining a constant tension in a cable, said apparatus comprising:

a travelling block;

a pair of cylinder/piston assemblies suitably connected by a bridge piece, said assemblies moving in unison'with each other through said bridge piece, said assemblies being supplied with energy from an external source;

a pair of connecting members for connecting said travelling block to said cylinder/piston assemblies;

a first drum, said drum being mounted for rotation on said pair of connecting members, said first drum having suitable means for connecting one end of the cable thereto, and said first drum having a cable supporting surface having a non-constant radius; and

a transmission means connecting the first drum to said bridge piece, said transmission means including a second drum coupled to the first drum to rotate in combination therewith and a cable coupled between the bridge piece and the second drum to rotate said second drum when said pistons move.

surface is in the shape of a spiral.

' l t III 

1. An apparatus for maintaining a constant tension in a cable, said apparatus comprising: a cylinder/piston assembly supplied with energy from an external source; a cable drum, said drum being mounted for rotation about a central axis and having a supporting surface for the cable, said surface having a non-constant radius, one end of the cable being connected to the drum and the part of the cable adjacent to said end passing over the supporting surface of the drum; and a transmission means connecting said piston to the cable drum, said transmission means being suitable to convert longitudinal displacements of the piston into rotation of the cable drum about its central axis.
 2. The apparatus of claim 1 wherein the supporting surface is in the shape of a spiral.
 3. The apparatus of claim 1 wherein the energy source is high pressure gas.
 4. The apparatus of claim 1 wherein said apparatus is mounted on a floating structure that moves vertically in response to wave action and said cable extends from said floating structure to a position that does not move in response to wave action.
 5. An apparatus for maintaining a constant tension in a cable, said apparatus comprising: a first drum having suitable means for connecting one end of the cable thereto, said first drum having a cable supporting surface having a non-constant radius; a cylinder/piston assembly supplied with energy from an external source; a transmission means connecting the first drum and piston of said cylinder/piston assembly, said transmission means comprising a second drum coupled to the first drum to rotate in combination therewith; and a cable coupled between the piston and the second drum to rotate said second drum when said piston moves.
 6. The apparatus of claim 5 wherein the supporting surface is in the shape of a spiral.
 7. The apparatus of claim 5 wherein the energy source is high pressure gas.
 8. An apparatus for maintaining a constant tension in a cable, said apparatus comprising: a travelling block; a pair of cylinder/piston assemblies suitably connected by a bridge piece, said assemblies moving in unison with each other through said bridge piece, said assemblies being supplied with energy from an external source; a pair of connecting members for connecting said travelling block to said cylinder/piston assemblies; a first drum, said drum being mounted for rotation on said pair of connecting members, said first drum having suitable means for connecting one end of the cable thereto, and said first drum having a cable supporting surface having a non-constant radius; and a transmission means connecting the first drum to said bridge piece, said transmission means including a second drum coupled to the first drum to rotate in combination therewith and a cable coupled between the bridge piece and the second drum to rotate said second drum when said pistons move.
 9. The apparatus of claim 8 wherein the supporting surface is in the shape of a spiral. 